Gina Miller, an investment manager and anti-Brexit campaigner, is leading a legal challenge over Theresa May’s right to trigger article 50.
Photograph: Getty

Amid death threats and intensifying political disagreement, three of the most senior judges in England and Wales will hear claims that the government cannot trigger Brexit without parliamentary approval.

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Scores of QCs and lawyers will cram into court four on Thursday, the largest in London’s Royal Courts of Justice, to hear two and a half days of argument that could decide how – or conceivably even whether – the UK leaves the EU.

The legal dispute is over who has authority to notify Brussels formally that Britain is withdrawing under article 50 of the treaty on European Union (TEU) – parliament or ministers.

Article 50 states that any member state may leave “in accordance with its own constitutional requirements”, an undefined term that has allowed both sides to pursue rival interpretations.

The case could open deep rifts in the consensus over the UK’s unwritten constitution. If the judges concluded that MPs should decide, the majority might not be in favour of leaving.

The government maintains that the decision to depart has been taken by the referendum on 23 June and that its executive powers, under the royal prerogative, are sufficient for David Davis, the Brexit secretary, to give notice on behalf of the cabinet.

The attorney general, Jeremy Wright QC, who will lead the government team, also asserts that initiating withdrawal does not change any UK laws and that the consequences will be subject to future negotiation and parliamentary scrutiny. The government’s lawyers also argue that the courts are “ill-suited” to decide on the matter.

David Davis, secretary of state for exiting the European Union. Photograph: Andy Rain/EPA

But the teams of lawyers deployed by the claimants, interested parties and interveners, some of whom are crowd-funded, counter that the referendum was merely consultative and that a decision to leave has not yet been taken.

Parliament alone, they argue, has the power to decide. Davis, one skeleton argument explains, “may only notify such a decision to the European council under article 50 (2) TEU once he has been properly authorised to do so by an act of parliament”.

The submission also quotes approvingly from the Bill of Rights 1689, a piece of legislation revered by Eurosceptics, that it “expressly prohibits the use of the prerogative in circumstances where its exercise would ‘suspend’ or ‘dispense’ statutory law”. Subsequent legislation has further restricted the extent of the royal prerogative.

The two main claimants leading the judicial review challenge are Gina Miller, a businesswoman and philanthropist, and Deir dos Santos, a hairdresser. Both are British nationals.

Miller said the challenge was not an attempt to overturn the referendum decision, telling BBC Radio 4’s Today programme: “We are all leavers now.”

The case, she said, was intended to “answer a fundamental legal question about the powers that can be used by the prime minister and whether they can side-step parliament”.

The People’s Challenge, one of the interested parties representing individuals who live in England, France, Gibraltar, Northern Ireland, Scotland and Wales, has to date raised more than £150,000 from more than 4,000 supporters to finance the case.

There have been demonstrations outside the London office of the Mishcon de Reya law firm by opponents holding placards declaring: “Invoke article 50 now” and “Uphold the Brexit vote”.

Anneli Howard, a barrister at Monckton Chambers working with Mishcon de Reya, said this week that “a number of death threats” and a large amount of online abuse had been directed against those leading the legal challenge.

At a preliminary hearing, Sir Brian Leveson ordered that the identities of some of the claimants should be withheld to protect them. “Apart from the commission of a criminal offence, there’s a real risk that behaviour of this type is a contempt of court,” he said.

The high court hearing in London is before the lord chief justice, Lord Thomas of Cwmgiedd, the newly appointed master of the rolls, Sir Terence Etherton, and Lord Justice Sales.

The claimants will make their case on Thursday. The government is due to respond next Monday and further argument may go on into Tuesday. Judgment is expected to be reserved.

A similar claim has been heard in Northern Ireland, stressing additionally the complexities of devolution legislation, which, it is alleged, would prevent withdrawal from the EU without further consultation. That judgment has also been reserved.

Whoever loses each case is expected to appeal directly to the UK’s supreme court in London in December. Because of the short period available before March, when Theresa May says she will trigger Brexit, provision has been made for the claims to leapfrog the court of appeal and go directly to the supreme court.

The most tangled outcome would be for the judges to refer the matter to the European Union’s court of justice (ECJ) in Luxembourg seeking clarification on what is meant by “in accordance with its own constitutional requirements”. None of the parties is urging that outcome, which would in effect hand over to European judges fundamental questions about British sovereignty.

A woman holds a sign to show support for the European Union at an anti-Brexit protest in Whitehall, London, in July. Photograph: Daniel Leal-Olivas/PA

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In a parliamentary debate on Wednesday over MPs’ scrutiny of Brexit, Dominic Grieve QC, the Conservative former attorney general, pointed out there was a convention that major treaty changes had to be put to the House of Commons for an affirmative vote. Although that was only a convention, he said, conventions must be followed. The notion that governments resign after losing a vote of confidence may only be a convention, he added, but MPs would be very surprised if that were ignored.

Before the hearing, Wright, said: “The country voted to leave the European Union in a referendum approved by act of parliament. There must be no attempts to remain inside the EU, no attempts to rejoin it through the back door, and no second referendum. We do not believe this case has legal merit. The result should be respected and the government intends to do just that.”

Dominic Raab, the Conservative MP and prominent leave campaigner, called the challenge “a rather naked attempt to steal the referendum by the back door”.

“I don’t think that it’s right that a fund manager with deep pockets and friends in high legal places gets to go to court and try to frustrate that decision,” he told the BBC. “The vast majority of people in this country, leave or remain, would think it was outrageous that a small group of people could go to court to overturn a decision made by 33 million people.”

The main claimants, interested parties and interveners are:

Gina Miller, a UK investment manager. She is represented by the law firm Mischcon de Reya, Lord Pannick QC and Rhodri Thompson QC.

Deir Dos Santos, a hairdresser who holds UK and European citizenship, is represented by the law firm Edwin Coe, Dominic Chambers QC and Jessica Simor QC.

The People’s Challenge, a crowdfunded initiative, represented by the law firm Bindmans, Helen Mountfield QC and Gerry Facenna QC.

AB and a child represented by the law firm Bhatia Best and Manjit Gill QC.

Fair Deal for Expats, a group formed of British expatriates living abroad in 10 EU states, represented by the law firm Crofts Solicitors and Patrick Green QC.

The defendant resisting the actions is the secretary of state for exiting the EU, David Davis, who is represented by the attorney general, Jeremy Wright QC, James Eadie QC and Jason Coppel QC.

• This article was amended on 13 October 2016 to remove part of an indirect quote as the information in it was found to be incorrect.